Organic-Free Synthesis of CHA-Type Zeolite Catalysts
for the Methanol-to-Olefins Reaction

Chabazite (CHA)-type zeolites are
prepared from the hydrothermal conversion of faujasite (FAU)-type
zeolites, dealuminated by high-temperature steam treatments (500–700
°C), and evaluated as catalysts for the methanol-to-olefins (MTO)
reaction. The effects of temperature and partial pressure of water
vapor during steaming are investigated. Powder X-ray diffraction (XRD)
and Ar physisorption data show that the steam treatments cause partial
structural collapse of the zeolite with the extent of degradation
increasing with steaming temperature. <sup>27</sup>Al MAS NMR spectra
of the steamed materials reveal the presence of tetrahedral, pentacoordinate,
and octahedral aluminum. NH<sub>3</sub> and <i>i</i>-propylamine
temperature-programmed desorption (TPD) demonstrate that steaming
removes Brønsted acid sites, while simultaneously introducing
larger pores into the CHA materials that make the remaining acid sites
more accessible. Acid washing the steamed CHA-type zeolites removes
a significant portion of the extra-framework aluminum, producing an
increase in the bulk Si/Al ratio as well as the adsorption volume.
The proton form of the as-synthesized CHA (Si/Al = 2.4) rapidly deactivates
when tested for MTO at a reaction temperature of 400 °C and atmospheric
pressure. CHA samples steamed at 600 °C performed the best among
the samples tested, showing increased olefin selectivities as well
as catalyst lifetime compared to the unsteamed CHA. Both lifetime
and C<sub>2</sub>–C<sub>3</sub> olefin selectivities are found
to increase with increasing reaction temperature. At 450 °C,
CHA steamed at 600 °C reached a combined C<sub>2</sub>–C<sub>3</sub> olefin selectivity of 74.2% at 100% methanol conversion,
with conversion remaining above 80% for more than 130 min of time-on-stream
(TOS) before deactivating. More stable time-on-stream behavior is
observed for 600 °C-steamed CHA that underwent acid washing:
conversion above 90% for more than 200 min of TOS at 450 °C with
a maximum total C<sub>2</sub>–C<sub>3</sub> olefin selectivity
of 71.4% at 100% conversion.